1. Field of the Invention
The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations, and to the manufacture of such bits.
2. Description of Related Art
In a first aspect the invention relates to drill bits of the kind comprising a main body part having a shank for connection to a drill string, an end face, an internal passage for supplying drilling fluid to said end face, a plurality of blades extending from said end face outwardly and longitudinally of the central axis of rotation of the bit, and a plurality of cutters mounted on each said blade, each blade comprising a central metal core at least partly surrounded by solid infiltrated matrix material.
The solid infiltrated matrix material is formed by a powder metallurgy process in which a hollow mould is provided in the required configuration of the outer surface of the bit body, or a part thereof. The main body part of the bit is located within the mould and the spaces between the main body part and the internal surfaces of the mould are packed with powdered hard material, usually tungsten carbide, which is then infiltrated with a molten metal alloy, such as a copper alloy, in a furnace so as to form a hard solid infiltrated matrix. (The term "solid infiltrated matrix" will be used herein to refer to the whole solid metallic material which results from the above process, i.e. tungsten carbide or other hard metal powder surrounded by solidified alloy which has been caused to flow, when in the molten state, into the mass of hard metal powder. The term "matrix" is the term commonly used for such material in the drill bit industry, notwithstanding the fact that, in strict metallurgical terms, it is the infiltration alloy alone which forms a matrix, in which the hard metal particles are embedded.)
In a drill bit of the above-mentioned kind the matrix material, which is highly resistant to erosion and abrasion, provides the outer surface of the blades and, usually, at least a part of the outer surface of the main body part and end face of the drill bit. However, the cast matrix material is comparatively brittle and the central metal core of each blade, which will normally be of a more ductile material, provides reinforcement of the matrix material. This is particularly desirable with bit designs where the stand-off of the blades from the end face is large.
A drill bit of this kind is disclosed in U.S. Pat. No. 4,667,756. In the arrangement described in that specification the main body part of the bit comprises a metal mandrel and each of the blade reinforcement cores is tack-welded, glued, press fitted, brazed or otherwise attached to the metallic mandrel. When the matrix is cast each core, and its attachment to the mandrel, is wholly enclosed with the infiltrated matrix material.
The present invention provides, in a first aspect, an improved method of manufacturing a drill bit of the kind first referred to above.